Highly Sensitive Detection of Infrared Photons by Nondegenerate Two-Photon Absorption under Midinfrared Pumping

We demonstrate highly sensitive photon counting in the infrared region based on two-photon absorption (2PA) in a silicon avalanche photodiode, where the required photon energy for inducing effective conductivity is provided by an intense midinfrared (MIR) field at 3 µm. The used MIR pumping scheme can not only benefit from the enhanced 2PA coefficient in the nondegenerate regime, but also eliminate the detrimental background noises due to the pump harmonic excitation of the pump. Consequently, the enhancement factor for the signal counting rate unprecedentedly reached about 105 with input infrared pulses at the femtojoule level. Additionally, the noise equivalent power is substantially improved by 2 orders of magnitude in comparison to conventional schemes with near-infrared pumping. Therefore, the presented configuration might provide an alternative to realize sensitive infrared detection and imaging with desirable features of room-temperature operation, no phase-matching requirement, and broadband responding window, which can find a variety of applications, including remote ranging, sensitive sensing, biochemical imaging, and trace spectroscopy.